Chapter 6

Light and Lighting Control in Greenhouses

Daily Light Integral

Daily light integral (DLI) is not a complex idea; all this refers to is the total amount of light received during a 24 hour period–it is a combination of not only the light intensity, which changes over the course of day, but also the duration of the light period. Daily light integral tells us how much plant usable light (expressed as PAR: μmol·m-2·s-1) the crop received inside the greenhouse during a 24-hour period. It is expressed as moles of light (mol) per square meter (m-2) per day (d-1), or, as we will describe it here, “moles/day”). Plant growth, development, rate of maturation, yield, and quality depend on DLI and provide information for determining when supplemental lighting or shading may be needed.

DLI Requirements for Greenhouse Crops

DLI levels will vary with plant species, but some general rules of thumb apply.

Measuring Daily Light Integral

There are three ways to measure the DLI in the greenhouse: 1) foot-candle meter, 2) quantum sensors, and 3) DLI maps.

Foot-candle Meter

Determine the average number of foot-candles (fc) per hour using a foot-candle meter (See Figure 6.1), add the fc readings, and then divide this sum by 24 (from midnight to midnight).

Quantum Sensor

The most accurate method is to use a light quantum sensor (See Figure 6.2) connected to a data logger or computer. The light sensor measures instantaneous light intensity (preferably in μmol.m-2.s-1) at some defined interval (such as once every 10 minutes), which measures and calculates the DLI your greenhouse crops are receiving. The instrument records light intensity, performs the mathematics, and outputs the DLI value in mol·m-2·d-1 (moles per day).

DLI Maps

The DLI at your location can also be estimated using DLI maps (See Figure 6.3) that were developed by Jim Faust at Clemson University. Faust determined the average DLI outdoors during every month for the contiguous United States. The values outdoors need to be converted into realistic values inside the greenhouse, which requires a light sensor to determine light intensity outdoors and inside the greenhouse at noon on a clear day.

Supplemental Lighting

Under light-limiting conditions (such as during the winter in temperate climates), most greenhouse crops benefit from supplemental lighting.  The practice of using high-intensity discharge (HID) sodium vapor or metal halide lamps to supplement natural sunlight during periods of inclement weather or short days allows growers to increase productivity and plant quality.

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